The cytotoxic response to TNP-modified syngeneic cells is small compared to the cytotoxic response to alloantigens. Limiting dilution analyses indicate that the number of lymphocytes in C57BL/6J(B6) lymph nodes responding to DBA/2J(D2) and B6-TNP are 1199 and 40 per 106 cells, respectively. These cytotoxic T lymphocytes (CL) produced as a result of stimulation of B6 lymphocytes with B6-TNP also lyse D2-TNP targets. Clonal analyses show that this cross-reactivity is a result of strong recognition of D2-TNP targets by 40% of B6-TNP specific CL and not relatively weak recognition of D2-TNP by all B6-TNP specific CL. When (C57BL/6J × DBA/2J)F1 mice (BDF1) were stimulated with BDF1-TNP, about 67% of the cytotoxic clones that lysed B6-TNP also lysed D2-TNP target cells; clones specific for either B6-TNP or D2-TNP were also produced. Cold target competition studies suggest that the higher degree of cross-reactivity in the latter case cannot be explained on the basis of the existence of antigenic specificities unique only to the F1 hybrid cells. Nor is the anomalously high F1 cross-reactivity due to the absence of parental antigenic specificities on the F1 cells. One possible explanation for these data is that the repertoire of cytotoxic precursors of the F1 hybrid for TNP-modified self antigens may be different from the sum of the repertoires for the parents.